The invention relates to a telecommunications antenna which is installed on a geosynchronous satellite and is intended to relay communications over an extensive territory.
A geosynchronous satellite which carries a send antenna and a receive antenna, each of which has a reflector associated with a multiplicity of radiating elements or sources, is used to provide communications over an extensive territory, for example a territory the size of North America. In order to be able to re-use communications resources, in particular frequency sub-bands, the territory to be covered is divided into areas and the resources are assigned to the various areas so that when one area is assigned one resource adjacent areas are assigned different resources.
Each area has a diameter of the order of several hundred kilometers, for example, and its extent is such that, to provide a high gain and sufficiently homogeneous radiation from the antenna in the area, it must be covered by a plurality of radiating elements.
FIG. 1 shows a territory 10 covered by an antenna installed on board a geosynchronous satellite and n areas 121, 122, . . . , 12n. This example uses four frequency sub-bands f1, f2, f3, f4.
The area 12i is divided into several sub-areas 141, 142, etc. Each sub-area corresponds to one radiating element of the antenna. FIG. 1 shows that some radiating elements, for example the radiating element 143 at the center of the area 12i, correspond to only one frequency sub-band f4, while others, like the radiating elements at the periphery of the area 12i, are associated with a plurality of sub-bands, i.e. the sub-bands which are assigned to the adjacent areas.
FIG. 2 shows a prior art receive antenna for a telecommunications system of the above kind.
The antenna includes a reflector 20 and a plurality of radiating elements 221, . . . , 22N close to the focal plane of the reflector. The signal received by each radiating element, for example the element 22N, is passed first through a filter 24N intended in particular to eliminate the (high-power) send frequency, and then through a low-noise amplifier 26N. The signal at the output of the low-noise amplifier 26N is split into several parts by a splitter 30N, possibly with coefficients that can differ from one part to another; the object of this splitting is to enable a radiating element to contribute to the formation of more than one beam. Thus an output 321 of the splitter 30N is assigned to an area 34p and another output 32i of the splitter 30N is assigned to another area 34Q.
The splitters 301, . . . , 30N and the adders 36P, . . . , 36q intended to define the areas are part of a device 40 referred to as a beam or pencil beam-forming network.
The beam-forming network 40 shown in FIG. 2 includes a combination of a phase-shifter 42 and an attenuator 44 for each output of each splitter 30i. The phase-shifters 42 and the attenuators 44 modify the radiation diagram, either to correct it if the satellite has suffered an unwanted displacement or to modify the distribution of the terrestrial areas.
Also, each low-noise amplifier 26N is associated with another low-noise amplifier 26xe2x80x2N which is identical to it and which is substituted for the amplifier 26N should it fail. To this end, two switches 46N and 48N are provided to enable such substitution. It is therefore necessary to provide telemetry means (not shown) for detecting the failure and telecontrol means (also not shown) to effect the substitution.
An antenna system of the type shown in FIG. 2 includes a large number of low-noise amplifiers, phase-shifters and attenuators. A large number of components is a problem on a satellite because of their mass. Also, a large number of phase-shifters 42 and attenuators 44 causes reliability problems.
The invention significantly reduces the number of low-noise amplifiers, phase-shifters and attenuators.
To this end, a receive antenna according to the invention includes:
at least one first Butler matrix, each input of which receives the signal from a radiating element and each output of which is associated with a low-noise amplifier in series with a phase-shifter and preferably with an attenuator,
a second Butler matrix which is the inverse of the first Butler matrix and has a number of inputs equal to the number of outputs of the first Butler matrix and a number of outputs equal to the number of the inputs of the first Butler matrix, the outputs of the second Butler matrix being combined to form the area beams, and
control means for controlling the phase-shifters and, where applicable, the attenuators, to correct or modify the beams.
In a Butler matrix, which is made up of 3 dB couplers, the signal at each output is a combination of the signals at all the inputs, but the signals from the various inputs have a particular phase, different from one input to another, so that the input signals can be integrally reconstituted, after passing through the inverse Butler matrix, followed by amplification and phase-shifting, and where applicable attenuation.
The number of outputs of the first Butler matrix is preferable equal to the number of inputs. In this case, the number of low-noise amplifiers is equal to the number of radiating elements, whereas in the prior art, as shown in FIG. 2, the number of low-noise amplifiers is twice the number of radiating elements. Furthermore, the number of phase-shifters is also equal to the number of radiating elements, whereas in the prior art the number of phase-shifters and attenuators is significantly greater, because the output signal of a radiating element is split and the phase-shifting and the attenuation 42, 44 are applied to each channel of the beam-forming network.
Controlling the phase-shifters in series with the low-noise amplifiers to correct or modify the beams is particularly simple in a receive antenna according to the invention.
Because Butler matrices are used, if a low-noise amplifier fails the signal is reduced uniformly at all the outputs.
To reduce the effect of an amplifier failure on the output signals, in one embodiment the low-noise amplifier which is associated with each output of the first Butler matrix includes a plurality (for example a pair) of amplifiers in parallel, for example interconnected by couplers. In this case, the degradation due to failure of only one of the two amplifiers of a pair is half or less than that if a single amplifier were associated with each output.
It can be shown that the degradation is equal to xe2x88x920.56 dB if 8th order Butler matrices are used with a pair of amplifiers in parallel associated with each output. The degradation is xe2x88x920.28 dB with 16th order Butler matrices and with a pair of amplifiers associated with each output of the first Butler matrix.
One embodiment uses a plurality of associated two-dimensional matrices, for example matrices in different planes, so that each signal received by a radiating element is distributed over nxc3x97n low-noise amplifiers, n being the order of each two-dimensional matrix. In one example n=8 and in this case each signal received by a radiating element is distributed over 64 low-noise amplifiers. In this example, if only one amplifier is associated with each output, failure of one amplifier leads to a loss of only xe2x88x920.14 dB.
The invention equally applies to a send antenna with a similar structure. In this case, the inputs of the first Butler matrix receive signals to be sent and the outputs of the second Butler matrix are connected to the radiating elements. Power amplifiers are provided for send antennas instead of low-noise amplifiers, of course.
In one embodiment that applies to sending and receiving, one of the Butler matrices and the beam-forming network constitute a single device.
It is already known in the art to use a structure with two Butler matrices for send antennas in order to distribute the send power over all of the power amplifiers, but in these prior art antennas the beams are corrected or reconfigured in the manner described for receive antennas with reference to FIG. 2. Accordingly, for send antennas, the invention reduces the number of phase-shifters, and where applicable attenuators, and also simplifies their control. Moreover, for receive antennas, as indicated above, the invention reduces the number of low-noise amplifiers (compared to prior art receive antennas).
Each pair of Butler matrices preferably corresponds to several areas. It is even possible to provide a single Butler matrix for all the areas. However, to simplify manufacture, it is preferable to provide a plurality of Butler matrices. In this case, some of the radiating elements can be assigned to two different Butler matrices. In this case, failure of an amplifier associated with a Butler matrix of a pair of Butler matrices degrades the signals for all of the beams associated with the corresponding Butler matrix. On the other hand, if there is no amplifier failure for the Butler matrix of the same pair, the sub-areas corresponding to the first matrix of the pair suffer attenuation, although there is no attenuation for the sub-areas of the second matrix of the pair.
To remedy this drawback, one embodiment of the invention controls the attenuators associated with a Butler matrix adjacent a matrix at least one amplifier of which has failed, in order to homogenize the send or receive powers.
Thus the invention relates to a receive (or send) antenna for a geosynchronous satellite of a telecommunications system intended to cover a territory divided into areas, the beam intended for each area being defined by a plurality of radiating elements, or sources, disposed in the vicinity of the focal plane of a reflector, the antenna being adapted to modify the locations of the areas or to correct an antenna pointing error. The antenna includes at least one first Butler matrix, each input (or output) of which is connected to a radiating element and each output (or input) of which is connected to a corresponding input of an inverse Butler matrix via an amplifier and a phase-shifter, the outputs (or inputs) of the inverse Butler matrices being associated with a beam-forming network, and the phase-shifters are controlled to displace the areas or to correct pointing errors, the first matrix and the inverse Butler matrix distributing the energy received by each radiating element over all of the amplifiers so that the effect of failure of one amplifier is uniformly distributed over all the output signals.
There is preferably an attenuator for equalizing the gains of the amplifiers in series with each amplifier and each phase-shifter.
In one embodiment, the antenna includes at least two Butler matrices with inputs (or outputs) connected to the radiating elements and at least one of the radiating elements is connected to an input of the first Butler matrix and to an input of the second Butler matrix.
In this case, it is preferable for the radiating element associated with two Butler matrices to be connected to the inputs (or outputs) of the two matrices via a 3 dB coupler and for an analogue coupler to be provided at the corresponding outputs (or inputs) of the inverse Butler matrices.
An attenuator can also be provided in series with each amplifier and phase-shifter; if an amplifier associated with a matrix fails, the attenuator attenuates the output signals of the other Butler matrix in order to homogenize the output signals of the two matrices.
In one embodiment, amplifiers are provided in parallel between each output (input) of the first Butler matrix and each corresponding input (output) of the inverse Butler matrix, and are associated by means of 90xc2x0 couplers, for example.
To correct an angular error and to repoint all the beams simultaneously, the phase-shifters preferably modify the slope of the phase front of the output signals of the first Butler matrix.
The inverse Butler matrix and the beam-forming network advantageously constitute a single system.
When an attenuator is provided in series with each amplifier, the amplifier preferably has a dynamic range less than 3 dB.
The Butler matrices are 8th order or 16th order matrices, for example.
In one embodiment, the antenna includes a first series of first Butler matrices disposed in parallel planes and a second series of first Butler matrices also disposed in parallel planes in a direction different from that of the first series, for example orthogonal thereto, to enable displacement of the areas or correction of pointing errors in two different directions and thus in all the directions of the area covered by the antenna.